Install the Postgrey package:
aptitude install postgrey
Configure the delay and message settings in /etc/default/postgrey:
POSTGREY_OPTS="--inet=127.0.0.1:60000 --delay=180" POSTGREY_TEXT="Temporarily deferred due to greylisting. Please try again later."
Restart postgrey:
/etc/init.d/postgrey restart
Enable grey-listing policy service in /etc/postfix/main.cf:
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, [...etc] reject_unauth_destination, check_policy_service inet:127.0.0.1:60000,
Restart Postfix:
/etc/init.d/postfix restart